1. Field of the Invention
The present invention relates to an optical disc (like a CD (compact disc) or a DVD (digital video disc)), and an optical disc apparatus which carries out at least recording or reproducing information on the optical disc.
2. Description of the Related Art
Recently, optical discs have been developed as a widely used recording media, the CD is a representative one of them. It is a reproducing only disc which contains sound or other signals recorded as digital data. At present, there are also CD-WO, which can be rewritten, and CD-RAM, which can be erasable.
As control methods of an optical disc rotating speed, there are CAV (Constant Angular Velocity) which has constant angular speed (attaching importance to access speed), and CLV (Constant Linear Velocity) which has constant linear velocity (attaching importance to recording capacity). A music CD, for example, adopts the CLV method for the purpose of reproducing music originally. Spiral tracks are scanned at a constant linear velocity.
In the case of a music CD (also termed a CD-DA (Digital Audio)) of a reproducing-only type, CLV control can be carried out by synchronizing a rotating speed with the reproducing frequency of digital data. However, in case of CD-WO or CD-RAM, the above mentioned CLV control can not be carried out because digital data is not initially recorded on the tracks. For such recording discs, like a CD-WO, grooves showing track position are wobbled with a constant cycle in correspondence to address information. The CLV control is carried out by reproducing a wobble signal from a tracking signal of the grooves.
For example, FIG. 10 shows an optical CD-WO disc 1. In order to form spiral tracks which carry reproducing and recording information, the grooves 2 which are the tracks, are formed with a predetermined pitch. They are wobbled in correspondence to address information. Lands 3 are located between the grooves 2 and are not used as tracks.
In more detail, as shown in FIG. 11, the address information, which comprises binary data, after being converted to bi-phase code, it is converted to a wobble signal by frequency modulation. In this situation, "0" of the address information is converted to "0,0" of the bi-phase code. "1" of the address information is converted to "1,0" of the bi-phase code. Further, a "0" of the bi-phase code is converted to a sine wave of frequency 21.05 KHz by the frequency modulation while a "1" of the bi-phase code is convened to a sine wave of frequency 23.05 KHz by the frequency modulation.
When an optical disc apparatus carries out recording and reproducing information on such an optical disc, the grooves 2 are scanned by the optical head. As the grooves 2 are wobbled by a frequency which is very different from a tracking error signal, it overlaps the tracking error signal and ATIP (Absolute Time In Pre-groove) wobble signal.
An address reproducing circuit 4 of the optical disc apparatus, as shown in FIG. 12, derives the ATIP wobble signal from the tracking signal by a band pass filter 5. If the ATIP wobble signal is digitized with a threshold value of 22.05 (KHz) by a frequency detector 6, bi-phase code is restored. Thus, address information is decoded from the bi-phase code, and can be used as a synchronizing signal to control a rotating speed of the optical disc 1, ensuring that the grooves 2 move relative to the optical head with constant linear speed.
An optical disc like the above mentioned CD has the address information recorded by wobbling tracks. Therefore, the recording area of the tracks is not used for recording the address information, and the recording area of the tracks is fully used for recording data.
However, with such an optical disc, it is impossible to record long time image data with high quality. Therefore, an increased recording capacity is desired. Though it has been considered to arrange tracks in high density, when arranging the wobbled tracks in high density, tracks which are adjacent to a laser spot (for detecting the tracks optically) interface. Therefore, the S/N (Signal-to Noise ratio) of the tracking signal is lowered.
To solve this problem, the present inventors have considered a method which seeks to compress the wobbling amplitude of the tracks in proportion to a arrangement pitch. In this situation, the C/N (Carrier to Noise ratio) of the wobble signal falls and decoding accuracy of the address information falls. In the present optical disc, it is impossible to increase recording capacity while maintaining the track arrangement in high density, with both good S/N of the tracking signal and good C/N of the wobble signal.